In the drilling process, a viscosity improver is usually added to improve the viscosity of low-solid drilling fluid, to ensure the drilling fluid has high viscosity and favorable rheological property. All drilling fluid viscosity improvers are water-soluble polymers with a long molecular chain. Viscosity improvers are often used also as shale inhibitors (coaters), filtrate reducers, and flow pattern improvers, etc., besides taking a viscosity improving effect. Hence, using a viscosity improver is often helpful for improving the rheological property of the drilling fluid and improving the stability of the well wall. Drilling fluid viscosity improvers are one of the most important auxiliary drilling agents. They are mainly categorized into natural vegetable gum viscosity improvers and synthetic polymer viscosity improvers, and take an important role in ensuring safe use of drilling fluids and cuttings carrying, etc. However, existing drilling fluid viscosity improvers, such as xanthan gum and 80A51, etc., can't meet the requirements of construction at a high temperature satisfactorily. In most viscosity improvers, the viscosity improving effect is degraded quickly or even lost completely at a temperature higher than 150° C. In the aspect of resistance to salinity, resistance to high-concentration calcium chloride is an urgent problem to be solved in the drilling fluid field. Among viscosity improvers commonly used at present, there is hardly any polymer that is resistant to calcium chloride at concentration higher than 15%.
For example, the calcium-resistant polymeric viscosity improver disclosed in CN102372818A and CN102464761A, which is obtained mainly by introducing sulfonated groups or hydrophobic groups into an acrylamide polymer, can only meet the requirements for use at low temperatures (lower than 100° C.) at calcium ion concentration not higher than 2,000 mg/L. Though the high-temperature resistant viscosity improver disclosed in CN101955564A, CN103113518A and CN102127401A has significantly improved resistance to high temperature and can be used at temperatures higher than 200° C., the salinity and calcium resistance property is not mentioned in the documents, and, conjectured from the monomers and preparation method used in the viscosity improver, the salinity and calcium resistance property is not improved substantively.
In summary, among viscosity improvers for drilling fluids commonly used at present, it is often unable to make a balance between high-temperature resistance property and salinity and calcium resistance property.